1. Field of the Invention
The present invention relates to the field of electrode materials, and more specifically, to a process for preparing a formulation comprising a carbon-deposited alkali metal oxyanion, as precursor of lithium-ion battery electrode coating slurry.
2. Description of the Related Art
Some alkali metal oxyanions, useful as cathode material, exhibit undesirably low electronic conductivity. One significant improvement to the problem of low electronic conductivity of some of these alkali metal oxyanion material, for instance of alkali metal phosphate, has been achieved with the formation of a carbon deposit on the surface of the material. Ravet has proposed using an organic carbon precursor that is pyrolysed onto the cathode material or its precursors, thus forming a carbon deposit, to improve electrical field at the level of the cathode particles. [See, e.g., U.S. Pat. No. 6,855,273, U.S. Pat. No. 6,962,666, U.S. Pat. No. 7,344,659, U.S. Pat. No. 7,815,819, U.S. Pat. No. 7,285,260, U.S. Pat. No. 7,457,018, U.S. Pat. No. 7,601,318, WO 2011/072397, WO 02/27823 and WO 02/27824)].
A milling step is generally used following the synthesis of an alkali metal oxyanion material to adjust the particle size distribution of the material in the range necessary for preparation of lithium-ion battery cathode. Nuspl (US 2007/0054187) has proposed a milling step using a rotor mill after carbon-deposition on LiFePO4 produced by a wet process, whereas Ravet (WO 08/062111) has proposed a milling step using a jet mill after solid-state synthesis, in a rotary oven, of C—LiFePO4 from precursors including an organic precursor as carbon-deposit source.
In the specific case of lithium iron phosphate, the quality of LiFePO4 carrying a deposit of carbon (C—LiFePO4) can deteriorate during exposure to air or during handling or storage [See, e.g. Ravet (WO 08/062111)]. This results in a detrimental change in the product or in the formation of impurities, which can subsequently exert a harmful effect on the cycling performance of the battery comprising the phosphate. Controlling the atmosphere around the product during handling, storage and use has been proposed for solving this problem.
The implementation of this process at an industrial scale presents some challenges as the properties of the end product may vary significantly from one batch to another, for instance depending on the nature of the precursors and/or of the product prior to the milling step.